Vibrational properties of a sodium tetrasilicate glass: Ab initio versus Classical Force Fields

TL;DR

This study compares vibrational properties of sodium tetrasilicate glass using classical force fields and ab initio methods, analyzing vibrational densities, mode characteristics, and Raman spectra to evaluate the accuracy of computational approaches.

Contribution

It provides a detailed comparison between classical and ab initio calculations of vibrational properties in sodium tetrasilicate glass, highlighting similarities and discrepancies across frequency ranges.

Findings

Low-frequency VDOS shapes are similar in both methods.

Discrepancies increase in intermediate and high-frequency ranges.

Calculated Raman spectra generally agree with experimental data.

Abstract

We have determined the vibrational properties of a sodium tetrasilicate (Na$_{2}$Si$_{4}$O$_{9}$) glass model generated by molecular dynamics simulations. The study has been carried out using a classical valence force fields approach as well as an {\it ab initio} approach in the framework of the density functional theory. The total and partial vibrational densities of states (VDOS) are presented, as well as some characteristics of the vibrational modes (participation ratios, correlation lengths). For the low-frequency bands below 500 cm${}^{-1}$, we find that the shapes of the two calculated VDOS as well as those of their corresponding partial VDOS are quite similar. For the intermediate- and high-frequency ranges, we observe larger discrepancies between the two calculations. Using the eigenmodes of the dynamical matrix we also calculate the polarized Raman spectra within the bond-polarizability approximation. We find an overall agreement between the calculated parallel polarized (VV) Raman spectra and the corresponding experimental spectrum. Regarding the perpendicular depolarized (VH) Raman spectrum, the comparison of the calculated spectra to the experimental data indicates a need for an adjustment of the VH bond-polarizability parameters.